Tuesday, January 31, 2012

Vavilov (0.8°S, 138.8°W), the 'relatively recent' 98 km crater straddles a crossroads in the violent timeline of the Moon's history and sports some of the Moon's highest elevations on the northern and western rim. Interestingly, Vavilov formed nearly on top of a similarly sized and much older crater whose rim is still visible as a semicircle immediately northeast. Both craters carved out the same unique notch in the west wall of Hertzsprung impact basin. Image from 160 kilometer wide field of view cropped from a LROC Wide Angle Camera monochrome (566 nm) mosaic stitched from eight June 3, 2010 orbital viewing opportunities averaging 76 meters per pixel with an incidence angle of 64.5° from 55 km altitude [NASA/GSFC/Arizona State University].

The elevation models of the Moon, built up during the record-breaking first 10,000 orbits by the Lunar Reconnaissance Orbiter (LRO), are finally allowing us to see the lunar surface in definitive detail. Naturally, this is especially true of the farside, invisible from Earth, and the polar regions. Even the vast highlands of the farside, unseen before 1959, have either been imaged at very wide angles or at low angle in part.

The Lunar Reconnaissance Orbiter Camera (LROC) in particular has been a spectacular success at imaging nearly half the Moon's surface at very high resolution and in surveying the entire Moon under a wide range of lighting conditions. One result is a highly accurate digital terrain model that just keeps getting better.

A weak attempt to represent a 100 degree wide panorama of the Vavilov interior and southern rim from its highest elevations situated along that crater's north rim. For the first time LROC is allowing us to imagine what that view might be, but it still does such a scene little justice to squeeze it into a 580 pixel-wide image [NASA/GSFC/ILIADS/Arizona State University].

Until LRO, many, but not all, of the wide angle views of the lunar farside have been focused on low resolution wavelength analysis, albedo, mineral and some low light relief. And even millions of laser altimetry measurements haven't matched the number of points recorded by LOLA during the LRO's present mission, already in lunar orbit far longer than any previous spacecraft. The record of the location of places photographed and measured hasn't been helped by the simple fact that no one really knew the Moon's actual shape and size with a high accuracy until Japan's Kaguya (SELENE-1) mission.

The Moon's highest elevations (10,761 meters) are now believed to be more than 600 km northwest of Vavilov, on the rounded wide rim of the crater Engel'gardt, but as we round the Moon's western rim, past Oceanus Procellarum and north of Mare Orientale where the farside highlands begin, and continue to proceed westward along the Moon equator the first very highest elevations encountered are on the north and western rim of Vavilov.

Easily among the highest elevations east of Engle'gardt East is on the upper reaches of the slumping wide north wall of Vavilov. We can only guess whether those heights could have once been much higher. The chaotic terraces of the western interior of Vavilov testify to a high degree of slumping, massive landslides underway since the crater formed. LROC WAC observation M130205287C (566 nm), orbit 4321, June 3, 2003; incidence 64.67° with a resolution of 77.11 meters per pixel, from an altitude of 55.31 km [NASA/GSFC/Arizona State University].

That Vavilov is deeply notched into the west-southwestern wall of the vast Hertzsprung impact basin is not something one can easily tell from Clementine (1994) albedo imagery, for example. So many craters with extended ray systems, like Jackson, overlap over the farside highlands, already bright for their relative lack of the mare-filled basins that dominate the nearside, that getting a gauge on elevations has remained elusive until the LRO mission. With the human eye alone its nearly impossible. But there is a reason why Vavilov is different, and higher, in one half than the other.

The west-southwest of Vavilov is not as stark a contrast in elevations as its north wall but the elevations are still respectable. The highest point along that rim is 9317 meters, among the Moon's highest places, and the high ejecta blanket, outside Hertzsprung on this side of the crater, tapers off less dramatically as well. The interior on the west side of Vavilov is more dramatically terraced, and this was probably not the original rim, its original circumference having collapsed, probably many times. The view seen in high detail in LROC Narrow Angle Camera (NAC) M151440688L shows a couple of kilometers-wide strip near this high elevation, and that along with other detailed images seem to show the process of slumping is still, slowly, underway. LROC WAC observation imaged at the same opportunity, LRO WAC observation M151440362C, orbit 7451 February 4, 2011; incidence angle 51.43° at a resolution of 81.2 meters per pixel from 58.55 km [NASA/GSFC/Arizona State University].

The Vavilov impact event was not the first to carve out a place on the wall of Hertzsprung. The crater is offset just a little to the southwest from the crater, of almost identical size, that first made the notch and first interrupted the full circle of the 590 km-wide Hertzsprung impact basin. Vavilov's progenitor came close to erasing it's sister sometime after, and all that remains of the older crater is a semicircle like a cup handle attached to Vavilov's northeast.

The LROC WAC-derived Digital Elevation Model (GLD100) brings Vavilov out of the glare, in false color. The terrain was already on the rise from the southeast before the Hertzsprung or Korolov (further westward along the farside equator) because the formation of the Moon's oldest, deepest and largest known South Pole Aitken impact basin, further southeast may have help to lift the whole wider area along its perimeter here 4 billion years ago. The uplift of the third ring of mountains around Hertzspring rose still higher, first interrupted here by the arrival of Vavilov D. The area carries the deep scars and secondary craters of what some believe to be the most recent mare-filled basin-forming impact at Orientale, to the southeast. Vavilov probably formed after that event, superimposed on all those more ancient happenings. Vavilov is about seven kilometers deep, from its floor to the heights on its north and west rims [NASA/GSFC/DLR/Arizona State University].

The high western side of Vavilov perched on the southwestern outer ring of Hertzsprung and, on closer examination, the scaring and secondary crater chains radiant from the energetic impact that formed Mare Orientale, straddling the Moon's west limb and visible on edge from Earth. The most influential morphology that lifted this area is mostly invisible from Earth, the wide and deep 4 billion year old South Pole Aitken (SPA) basin at lower left. Orthographic projection over the intersection of the Moon's equator and its 240th meridian east [NASA/GSFC/DLR/Arizona State University].

Vavilov was unfavorable placed for the Apollo mapping cameras, and not well situated, nor a priority, for the Lunar Orbiter photography before Apollo. Other than the polar regions, this area of the Moon received less attention than most other areas until Clementine, and then from a low-resolution experimental remote sensing standpoint. LRO has changed that, however, and so much else. We now know that the view, and from the standpoint of science, the excavation performed by the Vavilov progenitor warrants more attention.

Even from orbit Vavilov must be spectacular.

Courtesy of the NASA ILIADS application, the LROC 100m WAC Global Mosaic draped over the LOLA 128 px DEM (v.2), the simulated "orbital view" of Vavilov from 65 km over the center of Hertzsprung basin.

Full resolution detail from the LROC Featured Image, released January 31, 2012, showing contact between channelized impact melt and fractured melt
on the floor of Thales crater. LROC Narrow Angle Camera (NAC) observation M170218508L, orbit 10219, September 9, 2011; incidence angle 64.16° with a resolution of 0.47 meters per pixel from 39.12 km. Image field of view 290 meters. See the 700 meter field of view HERE
[NASA/GSFC/Arizona State University].

Many Copernican- and Eratosthenian-aged craters have substantial impact melt deposits within and surrounding them. Like so many other craters on the Moon, Thales crater (61.732°N, 50.284°E, ~31 km diameter) is remarkable in the diversity of geologic features observed in LROC NAC and WAC images. In the opening image, channelized impact melt flowed toward the crater floor from high up on the crater wall. The contact at which crater floor and wall meet is less distinct, but the change from channelized impact melt to fractured impact melt is representative of this boundary.

Tomorrow's Featured Image will explore why impact melt may fracture a specific way, but suffice to say, the morphology of the impact melt in today's Featured Image is spectacular!

LROC WAC monochrome (604 nm) mosaic of Thales (61.732°N, 50.284°E) swept up at the same time as the NAC frame source of the Featured Image, September 9, 2011; resolution roughly 55.6 meters per pixel. .Yellow rectangle notes location of the LROC Featured Image, at the contact
between crater wall and floor [NASA/GSFC/Arizona State
University].

It is likely that impact melt channels formed in the crater walls as hot impact melt flowed from higher elevation toward the crater floor, using pre-existing weaknesses and cracks in the target rocks created by the violent impact event. As more impact melt flowed downhill, these channels may have grown larger. On average, these channels are between ~25 - 40 m across. Some channels appear shallower than others, which may suggest that more melt flowed down the deeper channels, carving more material from them. Or, perhaps the impact melt cooled and solidified in the shallower channels prior to reaching the crater floor, thus explaining the apparent channel depth differences across this small region of the crater wall. Comparatively, the fractures in the impact melt on the crater floor represent the relief of stress during cooling as the melt solidified near the crater floor-crater wall contact. These fractures may have been caused by the impact melt "sticking" to the lowermost portion of the crater wall as it cooled, similar to a mixture of cornstarch and water drying out.

Why don't you go for an adventure on the floor of Thales crater and discover the amazing impact melt morphologies visible in the full LROC NAC frame?

Friday, January 27, 2012

The moon of today is a static orb with little to no internal activity; for all intents and purposes it appears to be a dead, dusty pebble of a world. But billions of years ago the moon may have been a place of far more dynamism—literally.

A new study of a lunar rock scooped up by Neil Armstrong and Buzz Aldrin during their Apollo 11 mission indicates that the ancient moon long sustained a dynamo—a convecting fluid core, much like Earth's, that produces a global magnetic field. The age of the rock implies that the lunar dynamo was still going some 3.7 billion years ago, about 800 million years after the moon's formation.

That is longer than would be expected if the lunar dynamo were powered primarily by the natural churning of a cooling molten interior, as is the case on Earth. The moon's small core should have cooled off rather quickly and put an end to any dynamo-generated magnetic field within a few hundred million years. So researchers may have to explore alternate explanations for how a dynamo could be sustained—explanations that depart from thinking of the lunar interior in terms of Earthly geophysics.

A standard-issue, Earth-like dynamo "would have died out on the moon much, much before 3.7 billion years ago," says Erin Shea, a graduate student in geology at the Massachusetts Institute of Technology and lead author on a study in the January 27 issue of Science. "We have to start thinking outside the box about what generates a lunar dynamo."

Using a high-resolution magnetometer, the researchers found that the lunar sample indeed formed in the presence of a magnetic field, perhaps even one as strong as Earth's magnetic field today. "What this sample tells us is that at some point the moon did have a dynamo," Shea says. "This magnetic field lasted much longer than we had considered before."

A similar paleomagnetic study in 2009 by some of Shea's co-authors demonstrated the presence of a lunar dynamo some 4.2 billion years ago. That is just at the cusp of what would be possible with an Earth-like dynamo driven by a cooling interior alone. "Even then it's not trivial," says Ian Garrick-Bethell, a planetary scientist at the University of California, Santa Cruz (U.C.S.C.), who was the lead author of the 2009 study.

A lovely combination of layered mare basalt, granular flow, and talus.
The top of the image is down-slope. LROC Narrow Angle Camera (NAC) observation M170694505L, orbit 10289, September 15, 2011; image field of view is 735 meters, pixel scale of 0.49 meters per pixel from 45.53 kilometers. See the much larger full sized LROC Featured Image HERE [NASA/GSFC/Arizona State
University].

Inside the southern rim of the crater Pytheas (20.55°N, 20.6°W) is a great combination of layered mare basalt, granular flow, and talus. In the bottom left hand corner of the Featured Image you can see the details of erosion where granular material fell away from the rest of the surface near the rim. The high reflectance (bright) tendril of material flowed in a narrow band over the layers of lower reflectance (darker) mare basalt, then, after clearing the basalt layers, finally spread into a wide cone of talus. Talus cones are common on the Earth, with some stunningexamples that may rival the Moon's beauty. On the Moon, talus deposits are created entirely by gravity, but on the Earth wind and water play a role in their formation.

A somewhat 'twisted' view of the larger slope context of the granular flows in the Featured Image from the LROC NAC frame. The apparent floor contact is, in fact, far from the crater's lowest elevations, on the opposite side of Pytheas [NASA/GSFC/Arizona State University].

A particularly detailed LROC Wide Angle Camera (WAC) monochrome (604 nm) 66 meter per pixel resolution image of Pytheas and Pytheas D directly to its north, the topography of its interior, and exterior ejecta blanket as well as albedo chevron, stitched from three sequential orbital viewing observations under an average 54.7° incidence angle from 46.8 kilometers, November 18 and 19, 2010 [NASA/GSFC/Arizona State University].

The same LROC WAC 604 nm mosaic at 50 percent (132 meter) of its original resolution offers a fuller view of the Pytheas chevron and other rays, apparently "downwind" from the Copernicus impact. The small crater to the west of Pytheas is Pytheas A. The inset shows elevation range of the Pytheas environs from LROC's versatile QuickMap [NASA/GSFC/Arizona State University].

Pytheas and the south-central Imbrium basin clearly had their topography and appearance affected by the relatively recent arrival of the Copernicus progenitor, 800 million years ago. LROC WAC 100m Global Mosaic overlaid upon LOLA digital elevation model (v.2) in the NASA ILIADS application [NASA/GSFC/Arizona State University].

Pytheas was a Greek geographer and explorer (circa 325 BC) from a Greek colony in what is now Marseilles, France. He is especially important to lunar geology since his report on Earth's ocean tides was probably the first to associate the tides with the phases of the Moon.

Thursday, January 26, 2012

Layers of mare basalt affected
the paths of granular material that flowed down the crater wall. The top of the image is down-slope. LROC Narrow Angle Camera (NAC) observation M157418698RE, orbit 8333, April 14, 2011; field of view 546 meters and the pixel scale is 0.4 meters/px from an altitude of 38.6 kilometers. View the full size LROC Featured Image HERE [NASA/GSFC/Arizona State University].

Sarah BradenLROC News System

The wall of Dawes crater (17.21°N, 26.32°E) contains sections of spectacular mare basalt layering. However, mass wasting, a geologic process where material moves downhill due to gravity, has started to partially cover these beautiful outcrops.

Granular flows started above the outcrop and then flowed down the interior crater wall. As seen in the Featured Image, the topography of basalt outcrop caused the flow to deviate into narrow paths, away from a simple path flowing straight down the crater wall. As the crater Dawes ages over billions of years, the mare basalt outcrop will eventually be completely covered with granular material due to slumping of the crater's walls and more mass wasting.

The full width of LROC NAC M157418698R
with the high resolution field of view in the January 25, 2012
LROC Featured Image set off by the yellow box. The slope of the
south-southeastern wall of Dawes, from floor (upper left) to rim (bottom) rises nearly 2 kilometers in elevation [NASA/GSFC/Arizona State University].

Through the last release of LROC Narrow Angle imagery, all but the center swath of Dawes has been photographed at mission-optimal high resolution.
The yellow arrow marks the location of the field of view in the
LROC Featured Image released January 25, 2012 and the yellow rectangle the
area swept up in the full LROC NAC observation [NASA/GSFC/Arizona State University].

Dawes has an asymmetrical
ejecta to match the asymmetry of its rim elevation and floor. The fan
of its ejecta blanket, still visible on the crater's
western flank, sweeps north and crosses 'under' the color contact separating
the mare of Tranquillitatis from the Serenitatis basin. Because that change is superimposed
over the Dawes ejecta the crater is older, at least, than the last time (the last of
many times) the ancient Serenity basin was flooded with impact melt [NASA/GSFC/Arizona State University].

Admiral Alan B. Shepard, Jr., USN
(1923-1998), the first American in Space
and fifth human to explore the lunar
surface as commander of Apollo 14 poses
with the third of six U.S. flags deployed on
the Moon, February 5, 1971 [NASA/JSC].

"When they have 13,000 Americans living on the moon, they can
petition to become a state," Gingrich said to applause at a speech on
Florida's Space Coast. "By the end of my second term, we
will have the first permanent base on the moon, and it will be American."

During the GOP presidential debate carried on CNN, Thursday, Jan. 26 at the University of North Florida in Jacksonville former Massachusetts Governor Mitt Romney ridiculed the idea as mere pandering to Floridians on the Space Coast suffering in a tough economy.

"If I were the CEO of a Fortune 500 company," Romney said, "and an aide came to me with the suggestion that we invest in a colony on the Moon I would fire him."

Former Senator Rick Santorum (R-PA) remained open minded, he claimed, while U.S. Rep. Ron Paul (R-TX) said government should be taken out of the way. Meanwhile former Speaker of the U.S. House Newt Gingrich (R-GA) reiterated an idea some have called 'grandiose' as necessary to inspire young Americans to study the sciences. He looked forward to a day, he said, when the Kennedy Space Center hosted six launches a day.

Somewhat lost in the crossfire of a heated presidential primary contest among U.S. Republicans fighting one another for the opportunity to unseat Barack Obama in November is a simple truth the candidates and their camps failed to grasp. Until 2009 establishing a permanent human presence on the Moon was official National Space Policy in the United States.

Wednesday, January 25, 2012

In space circles, the idea of offering incentive prizes to develop complex technology has some currency. Most notably, Republican presidential candidate Newt Gingrich recently advocated a prize-based incentive model coupled with a leaner NASA as an alternative to our currently stalled, government bureaucratic model of space operations. The incentive idea is behind the current Centennial Challenges program of NASA, which offers money for the demonstration of certain specified technologies or procedures. Presumably, Gingrich is speaking not of this existing program but about a vastly expanded prize structure, funded by the federal government, for significant milestones in humanity’s expansion into space.

This model structure harkens to early days of aviation when prizes for specific aeronautical achievement proliferated. Notable was the $25,000 Orteig Prize offered by New York hotelier Raymond Orteig for the first non-stop air flight between New York and Paris. Charles Lindbergh won the Orteig Prize in 1927 in his specially built Spirit of St. Louis. After this flight, probably due more to celebrity culture and the frenzy of fame rather than actual flight accomplishment, commercial aviation enjoyed a boom of popularity with the public and industry. In short, the prize offering succeeded in producing a PR stunt; the design features of Spirit of St. Louis were specifically optimized to permit Lindbergh to win the prize, not to advance aeronautical technology or establish commercial transatlantic flight operations.

Currently, the most visible prize structure for spaceflight is Peter Diamandis’ X-Prize Foundation, a private funding group that awards prizes for specific space-related goals. The first and most famous, the Ansari X-Prize founded in 1996, was offered to the first non-government group that could (within two weeks) twice launch and safely return to Earth a reusable, manned spacecraft. In 2004, the $10,000,000 X-Prize was won by Burt Rutan’s SpaceShipOne, funded by Microsoft’s Paul Allen. This vehicle used an innovative airborne launch system, a hybrid solid-liquid rocket engine and a “wing feathering” method for re-entry and return flight. Plans were immediately made to construct a commercial version of SpaceShipOne, to be sponsored and operated by Richard Branson’s Virgin Galactic organization.

However, since that prize-winning flight almost eight years ago, things have not proceeded smoothly. An explosion in 2007 destroyed the rocket fabrication facility and killed three workers. Virgin Galactic established an operations base in New Mexico on October 17, 2011. There is a passenger manifest backlog of 455 subscribers but as of this writing, not a single commercial passenger spaceflight has occurred.

Another current space prize is the Google Lunar X-Prize, offering a $20 million award for successfully landing a spacecraft carrying a high-definition imaging system and roving on the Moon at least 500 meters. Since its announcement in 2007, over 30 companies have registered to participate in the competition. Additional prize increments are awarded for other accomplishment, such as long range (> 5 km) roving, survival over a lunar night, and documentation of the presence of water in lunar soil. No lunar mission has yet been launched nor has any launch date been announced. The original expiration date for the lunar X-Prize was 2012 but was extended to the end of 2015.

An alternative incentive approach is milestone-based contracting. NASA’s Commercial Orbital Transportation Services (COTS) program awards government money to companies that meet specific milestones on previously announced timescales. That money is to be spent developing specific capabilities required for government needs. The reward at the end of this cycle is a performance-based government contract for launch services. However, under this government-sponsored incentive program, a commercial human spaceflight industry has yet to develop.

Bigelow Aerospace, a builder of private, “For Lease” space stations, recently laid off over one third of their workforce. Part of the problem is the lack of assured, commercially available access to their orbital stations. In 2004, Bigelow himself established and funded a $50 million prize to develop a commercial crew vehicle for orbital transport; the prize expired in 2010 without a single attempt at flight. Although rumor has it that Boeing is developing a spacecraft to serve private space stations, nothing has yet appeared, even in prototype form. Due to some unidentified technical issues, SpaceX has delayed the launch of the first flight of their Dragon cargo vehicle to ISS from early next month to an unspecified future date.

The simple glaring fact is the United States has no commercial human spaceflight industry. NASA’s attempt to encourage the development of such through COTS is floundering against some unpleasant realities: it is both very difficult and very costly to get into and back from space. The former drives up the cost, severely limiting potential markets. The latter stops not only imagined demand (such as space tourism) dead in its tracks but also real demand, such as government contracts for ISS crew access.

The hope of space prize enthusiasts for explosive growth in space similar to that seen in aviation innovation and industry following the winning of the Orteig Prize is unlikely to be realized. The problem is that spaceflight is a vastly more difficult field in which to participate than aviation. Many amateurs could and did fabricate aircraft in their garages and barns in the early decades of the last century. The First World War made surplus aircraft widely available at low cost, furthering the development of a robust early aviation industry. In contrast, no one has flown a surplus government space vehicle and “barnstorming” rockets do not exist, despite some imaginative depictions in Hollywood films.

Unfortunately, this is the space program we now have. No American human spaceflight flight systems exist and their development is dependent on the advent of a demand that has not yet materialized. Meanwhile, we comfort ourselves with fantasies about human missions to Mars. I appreciate and applaud Gingrich’s enthusiasm for space, a visionary attitude sorely lacking in most politicians. He needs to think carefully about how to incentivize the development of space and about the critical national needs served by our civil space program. Prizes seem attractive because of their historical role in stimulating a nascent aviation industry. But significant differences between aviation and spaceflight and our primitive level of development of the latter suggest that what worked before may not work now.

The impact crater in today's Featured Image rests on the edge of another crater known as Brayley G, however this crater is most likely volcanic! Brayley G is a beautiful volcanic vent located in the mare at 24.2°N, -36.4°E. In 2008, before LROC launched, we wrote about Brayley G in the Apollo Image Archive Featured Image.

Today we are proud to present a LROC NAC mosaic of the 3 km wide and less than 5 km long feature. Compare the new LROC NAC observation to images from Apollo 15 and 17 in the graphic below (or visit the Apollo Image Archive). Note how the differences in incidence angle highlight different features within Brayley G. The higher-incidence Apollo images highlight the morphology of the edges of the vent and the concentric faults. The lower-incidence LROC NAC image reveals the interior of Brayley G, which contains many boulders along the inside wall and more collapse features.

The same small crater (white arrow) in the context of the ancient Brayley G vent, from the full field of view seen in the LROC NAC mosaic released January 24, 2011. Below, a side by side comparison (original HERE) of Apollo 15 and Apollo 17 orbital mapping camera images. Because of LRO, it's now possible to see the interior of this volcanic feature [NASA/JSC/GSFC/Arizona State University].

So how do scientists tell the difference between a volcanic vent and an impact crater? Most lunar craters are bowl-shaped and circular depressions with raised rims. When an impact occurs it excavates material from below the surface and ballistically ejects that material outward from the point of impact. This process leaves a visible ejecta blanket around the crater rim. Over time, erosion and slumping of crater walls can degrade and eventually remove an elevated crater rim. Studying examples of small, recent impacts shows the link between these physical processes and the surface features they leave behind. Volcanic vents, on the other hand, are usually not circular and they do not have raised rims. While volcanic vents do not have impact ejecta blankets, they can be surrounded by a "halo" of pyroclastic material from a past eruption.

Brayley G is most likely a volcanic vent since it is has no elevated rim, is oblong in shape (not circular), and has no ejecta blanket. There are also concentric lines on the inside edge of Brayley G, which may be evidence of concentric faults, left by the partial collapse of the vent. Some depressions may also be formed by the collapse of a sublunarean cavity, such as an drained lava tube.

Tuesday, January 24, 2012

More than mere melt fracture, a narrow skylight among myriad melt fractures in the chaotic interior of the familiar nearside landmark Copernicus, at a resolution of 40 centimeters per pixel from 25.4 kilometers, August 18, 2011. LROC Narrow Angle Camera observation M168333206L; illumination from the southwest (bottom left) of directly overhead, angle of incidence 38.23° [NASA/GSFC/Arizona State University].

Joel RaupeLunar Pioneer

The LROC targeting team had already extensively mapped the interior of Copernicus before the brief period last August when the low point in the LRO polar orbit was reduced by half, sometimes below 25 kilometers. Copernicus might be the most photographed lunar crater after Tycho. Both of these relatively young impact craters are difficult to miss in any view of a waxing Moon seen from Earth. Both craters center on extensive and bright ray systems, and Tycho's being the youngest of the two can be picked out with the naked eye.

Copernicus, though larger than Tycho, seems less intense, almost smudged, and, indeed, it is more faded a more along in years, nearly old enough for the inevitable effects of space weathering to have gardened its face with optical maturity.

The interior, slumped terraced walls, rim and some of the ejecta blanket of Copernicus as seen in one of the very first LROC Wide Angle Camera (WAC) mosaics released by Arizona State University in early 2010. At its roughly 800 million year age Copernicus, namesake of the "revolutionary" polish astronomer Nicolas Copernic (1473-1543), has lent its name to the Copernican Age on the lunar timescale, that relatively recent and relatively sparse period of bombardment. Its interior is flatter in the north than at the south and features three central peaks. A bifurcated pattern to its rays system and topography has led some to speculate at least two progenitors of nearly equal size were involved in this impact event [NASA/GSFC/Arizona State University].

As LROC team member James Ashley was spotlighting the melt fractures of Jackson crater earlier this month we were already performing a survey of the same features on the floor of Copernicus, particularly within the crater's north-central and "more featureless" interior. The melt fractures within Copernicus seem more extensively gardened, and may have formed in a somewhat different way than those at Jackson. At Copernicus there seems to have been, or still may be, voids under the impact melt, and some evidence of what may be bubbling, places that seem to be half-submerged solid rubble on the floor of Tycho, for example, may be place where gases were trapped for a time, escaping after the impact melt had rapidly cooled and solidified.

The north central floor of Copernicus only seems less distinctive than the jumbled topography of its surroundings. Barely visible in this LROC WAC monochrome (643 nm) image is a web of fractures and channels throughout the most level terrain above. A 35 km-wide field of view from LROC WAC observation M147109260C, orbit 6813, December 16, 2010; resolution 60.3 meters per pixel at an incidence angle of 78° from 43.13 kilometers [NASA/GSFC/Arizona State University].

Both the north and south ends of this 42 meter-wide exposed fracture on the floor of Copernicus are nearly buried or are not quite as wide as this section. While its tempting to see the western side as an overhang, and despite the apparent differences in the opposing edges, the exposed rift is likely only slightly deeper and more shadowed. There is evidence enough for voids under the floor of Copernicus but the more obvious clues have been pulverized by space weathering and moon quakes since the crater's formation. LROC NAC observation M157730473L, orbit 8379, April 18, 2011; angle of incidence 24.32° on a field of view 270 meters wide at a resolution of 0.47 meters from 37.85 kilometers [NASA/GSFC/Arizona State University].

Is this 300 meter-wide feature on the floor of Copernicus a crater, a void shaken to collapse or a little of both? Apparent layering in rapidly cooling impact melt may be a result of differing arrival times of the melt. LROC NAC M168333260L [NASA/GSFC/Arizona State University].

The area of interest on the floor of Copernicus is distinguished by its melt fractures, many obvious and others mysterious. The traces of these fractures are often marked with pits, their openings too shadowed or narrow to measure - even discovering whether any are really open at all and what may lay below awaiting future exploration. Between the two "pits" appears to be a spot long collapsed - but what if anything actually collapsed? Also from LROC NAC M168333206L [NASA/GSFC/Arizona State University].

Zoom in, out and around this area of interest in Copernicus using the LROC QuickMap, HERE, in views like the one below.

Friday, January 20, 2012

A field of boulders casts long shadows on the south wall of northern mid-latitude crater Egede A.
Illumination from south-southwest at a 54.94° angle of incidence. Field of view is roughly 400 meters across. LROC Narrow Angle Camera (NAC) observation
M122137079L, LRO orbit 3135, March 2, 2010; resolution 0.49 meters from 42.45 kilometers. View the full size LROC Featured Image HERE [NASA/GSFC/Arizona State University].

As on Earth, that 'golden time' just before the sun dips below the western horizon produces spectacular shadow effects on the Moon, dramatically accentuating perceived surface roughness. Because the Moon has no atmosphere, its shadows are very sharply defined and the contrast between illuminated and shadowed areas is high. The Apollo astronauts often reported difficulty in judging distances to objects because without a hazy or dust-filled atmosphere to 'soften' the view, distant objects looked very similar to objects that were close up. Shadows are useful to planetary scientists doing remote sensing investigations because their length can help us determine the size of the object casting the shadow.

For example, here we see a family of boulders resting on the inner slope of Egede A crater (51.56°N, 10.45°E). Were this a horizontal surface, the shadow length of the largest boulder in the featured image would indicate its height to be approximately 61 m. The calculation is made by knowing the solar incidence angle and using a bit of high school trigonometry. Actually, however, the surface is not horizontal -- an 'early sunset' is produced for these boulders by the sloping crater wall, effectively exaggerating the boulder's height. Therefore the true height of this boulder is something less than 61 m. That means our sun angle is wrong for producing an accurate boulder size estimate. This also means that we would have to know the angle of the sloping crater wall in combination with the sun angle and shadow length to make our calculation. Planetary sciences teaches us to be cautious in our interpretations of what we think we see. Can you think of a way to determine the slope of the crater wall?

Yellow square shows the field of view of the Featured Image, near the rim of Egede A in the context of the full NAC M159080552L frame, a field of view approximately 2.5 kilometers wide [NASA/GSFC/Arizona State University].

Imagine that you're standing on the rim of this crater. The sun would still be relatively high above the horizon. Note how the surface beyond the crater in the context image below is in full sunlight. High overhead is the Earth, looking four times the diameter that the Moon does in our Earth sky. If you then held your hand up to block the sun, the rest of the heavens would be raven black and filled with stars. All your favorite constellations would be recognizable, just as if you were back on Earth, with no visible change in their positions relative to each other -- the distance between the Earth and the Moon simply isn't great enough to register a visible shift among star patterns. You had better find some shelter soon, because it will get very cold here when the sun finally sets!

Egede A in temporal context, seen here through the LROC Wide Angle Camera (WAC) in a monochrome (604 nm) mosaic stitched from observations swept up in orbits 3132 and 3134, orbital passes immediately before and after the the Featured Image (yellow box) was captured, March 2, 2010. LROC WAC observations M122130239C and M122143802C, with an average resolution of 59.9 meters per pixel from 42.03 kilometers altitude [NASA/GSFC/Arizona State University].

Note the faintly visible, light-colored ejecta pattern surrounding Egede A in the image below. This shows it to be a relatively young impact feature. The WNW-ESE trending crater chains to the north and south of Egede A are secondary impacts produced by ejecta from a much larger impact beyond the frame.

LROC WAC mosaic context image, showing greater relief in long shadows nearer to a true sunset. Note the trails of secondary craters and the outer edge of the ejecta blanket of the far older Aristoteles crater whose center is more than 120 kilometers away. Field of view is about 78 kilometers. View the larger LROC context image HERE [NASA/GSFC/Arizona State University].

Using a modest telescope (or Google Earth, or NASA's ILIADS application), if you find the Alpine Valley (Vallis Alps), the well-known spectacular fracture fault radiant northeast from Mare Imbrium, you can find Egede A. The crater is directly on the opposite end of a line running through the valley from the huge basin [NASA/ILIADS/ASU].

As molten rock cools, it shrinks and often cracks. In this case of impact melt ponded within the Jackson crater floor (22.18°N, 197.24°E), the cracking rate was so high that unfractured melt is almost more of an exception than a rule!

Radial and divergent patterns can be seen among the fracture sets that tell a story of the cooling history. The context image below shows a portion of their wider distribution.

As context for the January 18, 2012 LROC Featured Image (field of view
near where the impact melt inundating the crater floor emerges from
eastern wall slump; the white box) a long view north and up the steep
northeastern wall, nearly to the rim, courtesy of the digital elevation
model combined in Google Earth [NASA/USGS/ASU/JAXA/Google].

Overhead context for Featured Image, a field of view roughly 2.5 kilometers across from the wider LROC frame.View the full-size LROC context image HERE [NASA/GSFC/Arizona State University].

Solid objects in the melt, together with the 'shore' of the pond, appear to have influenced the way the cracks organized themselves as the melt cooled. Note how the fractures bend around or radiate from some of the positive relief features in the images above. These could be ejecta blocks or portions of the slumped crater walls in the melt that served to locally accelerate cooling. Their influence might thus be to 'seed' the stress field within the shrinking melt volume, helping some of the cracking to grow from these points, and ultimately resulting in the patterns we see today. Sagging along the shore can cause the cracking to parallel the shoreline. Any motion within the volume of melt, possibly influenced by late-stage additions of molten material, may also have contributed to the patterns observed here.

Further context, from 100 kilometers altitude, this square crop from a highly detailed HDTV still frame was captured by Japan's lunar orbiter SELENE-1 (Kaguya) in 2009 [JAXA/NHK/SELENE].

Ed Note: In a way opposite and contributing to the low optical visibility of the vast majority of similarly sized craters in the farside Highlands, Jackson is easier for the eye to see than most. Like Tycho on the nearside, there are a lot of craters of similar size and origin everywhere on the Moon. The difference is age. Like Tycho, the ray system of Jackson (and the materials its progenitor impact threw out) shows Jackson's "optical immaturity." To illustrate, below are two representations of the farside quadrant with the highest of the Highlands scoured by the Jackson impact, likely less than a half billion years ago.

Jackson stands out in this global montage of Clementine (1994) Ultra-Violet/Visible (UVVIS) wavelength photography designed to better map the Moon's albedo, more than a decade ago. Similar craters, basins and the Moon's highest elevations are nearly invisible [NASA/USGS/DOD].

A white arrow is needed to designate Jackson out from the pocked highlands and several otherwise invisible basins stand out with exceptional clarity in this view of nearly the same terrain as a representation of differences in elevation from the LROC Global Digital Terrain Model, developed using LROC Wide Angle Camera survey photography [NASA/GSFC/Arizona State University].